Help a newb with an amp + soundcard setup.

For music listening and dynamic/ortho headphones, that's probably your best bet indeed.

Analog motherboard audio generally isn't very clean. You'll hear hissing and buzzing with sensitive headphones. That's why people get sound cards or external DACs (along with any DSP or gaming features, if desired).

As for the Xonar Essence STX, it's said to perform better than external DACs in its price range or even more expensive, and the amp's said to be pretty powerful, too. I don't think you'll have to worry about an external amp unless you want to go electrostatic later on (because 'stats work too differently and require specialized amps). I don't have first-hand experience with the Essence STX, but if I wasn't so much into gaming, I'd probably have one as my card of choice.

I would recommend an external dac over any inboard device.

$300-500? If I knew you were willing to spend THAT much...

Coincidentally, it's also said to be the price range where external DACs actually start being legitimate improvements from internal sound cards, or so I've heard.

Someone advised me to scour AudiogoN for used PS Audio Digital Link models, preferably II or III if the budget permits. That, or a Rega DAC. (Keep in mind that I have no experience with these things and probably won't be buying one for a very long while. My budget's tight enough just trying to get a Stax amp and a better Lambda.)

The E9's said to be a good amp for the money, but the E7 is allegedly a worse DAC than the Essence STX if you actually look at the specs and don't need the portability. (Unless you use a laptop, which you most certainly were not if you were even considering internal sound cards.)